1. Field of the Invention
The present invention relates to trolling motors. More particularly, but not by way of limitation, the present invention relates to an apparatus and method for power management in a trolling motor.
2. Background of the Invention
Trolling motors are well known in the art. Generally speaking, a trolling motor is a relatively small electric motor coupled to a propeller for propelling a boat, or other water craft, at a relatively low speed. Typically, the electric motor and propeller are positioned at the bottom of a support column rotatably supported by a bracket which is attached to the boat. A control head located at the top of the support column houses electrical circuitry for controlling the motor. In addition, the control head may also house a steering motor and associated electrical circuitry to provide rotation of the support column to cause steering of the boat.
Trolling motors may include a number of additional features such as: mounting brackets which provide an operating position and a stowed position for safely positioning the trolling motor during operation of the boats engine or for transportation of the boat; reverse operation for propelling the boat backwards; various styles of foot pedals for controlling trolling motor features; autopilots; and the like.
A fisherman purchasing a trolling motor must typically weigh a number of conflicting factors. For example, a fisherman would typically desire the maximum thrust possible when purchasing a trolling motor. Unfortunately, the greater the peak thrust of a trolling motor, the larger the physical size of the motor, the more the motor must weigh, and, perhaps most importantly, the greater the cost of the trolling motor. This is due to the fact that the size and weight of a trolling motor are driven by the motor""s ability to dissipate heat at maximum torque. Presumably, motors will be operated at maximum torque for extended periods of time and, therefore, a motor must be designed to operate at maximum torque indefinitely without adverse effects.
While a trolling motor could be designed to produce high torque for transient, peak loads without a corresponding increase in size, weight, and cost, such a design has been impractical with present trolling motors since there are no safeguards against extended operation of the motor beyond the steady-state operating limits.
A further limitation of trolling motors involves the motor controller response to a stall, or near stall, condition. Trolling motors are known to be placed in a stall condition from a number of environmental factors such as, weed fouling, contact with the bottom, entanglement with ropes or lines, etc. During such stall conditions, the electrical current drawn by the motor would rapidly rise to unacceptable levels if not otherwise controlled. To protect the motor and controller from stall conditions, the controller typically employs a current limit circuit. While current limit circuits found on trolling motors serve to protect the motor and controller, they suffer from a number of limitation. For example: the propeller may immediately resume normal operation when a stall condition is removed causing the boat to lurch or endangering the user""s fingers and hands if the fouling condition was cleared by hand; the components which establish the current limit are typically unique to a particular model, thus necessitating a unique controller for each model of trolling motor; the current limit is xe2x80x9ctunedxe2x80x9d for a particular duty cycle (typically 100% duty cycle) and therefore may perform poorly at other speed settings; custom settings require physical modifications to a controller; and the transitions into and out of the current limiting mode are fixed by the particular design of the controller.
Similar problems exist with steering motor controllers. It is common for such controllers to likewise provide a current limit circuit to prevent damage to the motor and controller if the steering motor encounters a stall. A stall condition may occur when steering is attempted with the motor in a stowed position, if the motor is in contact with the ground, because of entanglement with a rope or line; etc. As with the trolling motor controller, the value of particular components which establish the current limit are calculated or determined empirically at the time the controller is designed and are typically peculiar to a particular trolling motor model. In addition, normal operation of the steering motor may resume immediately upon removal of the condition causing the stall which may result in sudden, unexpected turning of the boat.
Thus it can be seen that there is a need for a trolling motor having a motor controller and steering motor controller which manage the electrical current during peak loads or stall conditions in an intelligent manner.
The present invention provides a trolling motor having current based power management which resolves the problems and satisfies the needs identified above. The inventive trolling motor comprises: a support column rotatably supported from a mount; an electric motor mounted to the lower end of the support column; and a control head supported at the upper end of the support column. The control head houses a motor controller for controlling the electric motor and a steering system for rotating the support column to steer the boat during operation of the trolling motor. The steering system includes a reversible electric motor driven by a steering motor controller. Typically, both the motor controller and the steering motor controller are in electrical communication with a manual control device, such as a foot pedal, which directs operation of the trolling motor and steering system. Optionally, the motor controller and steering motor controller may also be in electrical communication with an autopilot, or the like, which directs the speed and steering of the boat automatically.
Preferably, the functions of both the steering motor controller and the trolling motor are incorporated into a common controller. A microprocessor or microcontroller (hereinafter xe2x80x9cmicroprocessorxe2x80x9d) provides a pair of pulse width modulated outputs driving a pair of power amplifiers to proportionally drive the trolling motor and the steering motor. The microprocessor monitors the current flowing through each electric motor with a current sensor, i.e., a current sense resistor, an electromagnetic current sensor, or the like, which produces an output representative of the current flowing through the motor in a form readable by the microprocessor. Alternatively, the microporcessor may provide a signal to a pulse width modulator rather than providing a pulse width modulated output directly.
In the preferred embodiment, a motor is calibrated with a lookup table, stored in the microprocessor""s memory, of allowable currents with respect to applied voltage. This table represents a profile of electrical current required operate the motor at any given speed. In operation, the microprocessor designates a motor speed and then refers to the lookup table to find allowable current. When the motor draws more electrical current at a given voltage than is allowed, the microprocessor reduces the applied voltage until the voltage/current relationship is within the limits prescribed by the table. Of paramount importance in a pulse width modulated control system, is the ability of the control circuitry to react instantly to current surges which could damage the power output components, resulting in failure. By constantly monitoring the output current and comparing the current with the maximum allowable current prescribed for any given voltage, the voltage can be adjusted instantly to prevent over-current failures of the drive electronics. The net result is that the microprocessor modulates the motor voltage to stay within the design parameters of the motor regardless of the possible changes of load caused by the above mentioned propeller entanglements or other forces that would impede motor rotation.
The maximum current table has the added benefit of tuning the performance of the motor to the propeller. Although not advisable, a fisherman can change the propeller on his trolling motor to an after-market propeller that will produce increased thrust. This has a deleterious effect on the motor since the motor current increases as the load is increased for any given rotational speed. With the present invention, the motor will still not perform outside of its designed parameters and, therefore, will remain unharmed.
Another subtle effect of the present invention is that the designer is free to design a motor which exceeds xe2x80x9ccontinuous dutyxe2x80x9d limitations with regard to heat dissipation. Since trolling motors typically are not operated in a continuous mode or as primary propulsion, the motor can be designed to operate in an intermittent fashion with the benefit of higher thrust available to the operator. Since current is constantly measured and the resistance of the motor is known, the operating temperature of the motor can calculated and compensated for. It should be noted that direct measurement of the motor temperature is unnecessary since the resistance of the windings and the applied current may be used to calculated motor heat dissipation (i2r losses). When the motor is used in the continuous mode for an excessive period of time, the maximum allowable voltage applied to the motor can be slowly reduced to a safe limit that will not exceed the heat dissipating capabilities of the motor and housing. This adjustment is made so subtly that the person operating the motor is unaware of the decrease in performance. The end result is greater performance in intermittent operation while, at the same time, providing continuous duty operation without the concern of overheating or damaging the motor.
It should also be noted that the gradual reduction in torque also conserves energy, extending the operational life of the battery between rechargings.
Furthermore, by comparing the measured current with respect to the expected current draw, the microprocessor can determine when the motor rotation is xe2x80x9cstalledxe2x80x9d completely. In a complete stall, and after a short period, the drive voltage is removed an the current drops to zero. Since the motor is stalled, it serves no purpose to continue applying power to the motor until conditions responsible for the stall have been removed. This may be by clearing the propeller of weeds or disentangling it from an obstruction. This feature also provides an extra margin of safety for the operator. Sometimes when a motor is entangled and the propeller no longer turns, the operator may be unaware that the motor is being electrically urged to rotate. A fisherman may pull the motor out of the water and attempt to clear the obstruction without turning off the motor. By having the motor automatically turn off in this condition, the operator cannot be inadvertently injured by the prop, were it to resume spinning once freed.
Similarly, maximum current values are stored for the steering motor. If the microprocessor detects electrical current flow through the steering motor which exceeds the predetermined limit, the motor drive is reduced to a very low setting. Upon detecting normal current flow, the microprocessor will gradually return to normal operation, likewise returning to higher torque output in a safe manner.
Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments.